linux-kernel - Re: [PATCH 1/3] arm64: rseq: Implement backend rseq calls and select HAVE

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Date:   Tue, 26 Jun 2018 11:31:14 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        arnd@...db.de, mathieu.desnoyers@...icios.com,
        peterz@...radead.org, paulmck@...ux.vnet.ibm.com,
        boqun.feng@...il.com, catalin.marinas@....com,
        peter.maydell@...aro.org
Subject: Re: [PATCH 1/3] arm64: rseq: Implement backend rseq calls and select
 HAVE_RSEQ

On Mon, Jun 25, 2018 at 06:54:43PM +0100, Will Deacon wrote:
>  /*
>   * Please add new compat syscalls above this comment and update
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 28ad8799406f..1eda9e1a1f4a 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -904,6 +904,7 @@ ENDPROC(el0_error)
>  ret_fast_syscall:
>  	disable_daif
>  	str	x0, [sp, #S_X0]			// returned x0
> +#ifndef CONFIG_DEBUG_RSEQ
>  	ldr	x1, [tsk, #TSK_TI_FLAGS]	// re-check for syscall tracing
>  	and	x2, x1, #_TIF_SYSCALL_WORK
>  	cbnz	x2, ret_fast_syscall_trace
> @@ -911,6 +912,7 @@ ret_fast_syscall:
>  	cbnz	x2, work_pending
>  	enable_step_tsk x1, x2
>  	kernel_exit 0
> +#endif
>  ret_fast_syscall_trace:
>  	enable_daif
>  	b	__sys_trace_return_skipped	// we already saved x0

I *think* this is ok, since we re-check the TIF bits in
syscall_trace_exit().

This does mean that we'd now always call audit_syscall_exit(),
regardless of TIF_AUDIT, but there are already cases when we call that
with TIF_AUDIT clear, so I think if that's a problem it's a latent bug.
Likewise for audit_syscall_entry().

It seems we're in the same boat as other architectures there,
regardless.

FWIW, for the entry bits:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.